1. Field of the Invention
The present invention relates to a photobase generator and a curable composition containing the photobase generator which are suitable for producing optical materials such as adhesives for optical devices, coating compositions for optical devices, resist materials, prisms, fiber optics, information recording media, filters and plastic lenses.
2. Description of the Prior Art
Plastic materials are recently finding wide application in various optical materials because of their lightweight, toughness and easiness of dyeing. A high refractive index is one of the characteristic features required for various optical materials. To obtain optical materials having a high refractive index, various episulfide compounds capable of providing optical materials having a refractive index of 1.7 or higher have been proposed (JP 9-71580A, JP 9-110979A and JP 9-255781A). However, most of the raw materials containing the proposed compounds are heat-curable and the resultant optical materials are greatly limited in their application. Therefore, a photo-curable raw material has been strongly demanded.
The photopolymerization of episulfide compounds are reported in U.S. Pat. No. 6,635,195, JP 2002-047346A, JP 2002-105110A, U.S. 2003/0022956 A1 and JP 2003-026806A. In the proposed photopolymerization, radical generators, acid generators, base generators, etc. are used as the initiators. Of the known generators, the base generators are considered to be most desirable because of their high activity to the polymerization of episulfide compounds. The study of photobase generators are reported in Kagaku Kogyo, vol. 50, pp 592-600 (1999) and J. Polym. Sci. Part A, vol. 39, pp 1329-1341 (2001). However, it is only a short time since the study of the photobase generators was begun, and no photobase generator having a practically acceptable activity has not been found until now.
The major problems of the photobase generators which have been hitherto developed are:    1. since the wavelength range of light absorbed by the photobase generator substantially overlaps that of episulfide compounds which ranges up to about 300 nm, the efficiency of generating base by photolysis is low; and    2. since the base generated by photolysis is weak in basicity, the curing by polymerization of the episulfide compounds is slow.